Many types of cutting machinery present a risk of injury because a user may accidentally contact the cutting tool during operation. For instance, every year, many thousands of people suffer amputation of one or more fingers while operating table saws. Various systems have been proposed to address this danger. For instance, U.S. Pat. No. 5,081,406 describes a system designed to detect dangerous proximity and trigger the shut down of a piece of equipment to avoid injury.
As another example, U.S. Pat. No. 3,858,095 describes a system that is purportedly able to stop a band blade on a band cutter machine used in the textile industry in about {fraction (1/200)}th of a second in the event contact between a user and the cutting blade is detected. However, it seems unlikely that the system set forth in the '095 patent could have worked to stop the blade in the time specified. In particular, assuming two 20-cm radiuses, 2-kg band wheels, the torque required to stop the wheels alone in {fraction (1/200)}th of a second is over 1000 N/M. The braking action is described as being provided by the winding of the drive motor and an electromagnetic brake. However, the motor for a band saw with 20-cm wheels might be a few horsepower at most, while supplying sufficient torque to slow just the wheels would require about 60 hp of power. In addition, the armature of a motor of a few horsepower would most probably have inertia greater than the wheels. Therefore, the motor would not even be able to stop itself in {fraction (1/200)}th of a second, let alone the wheels of the band saw. The electromagnetic brake would be of little more help. By way of example, a Stearns SM-250 electromagnetic clutch brake that sells for almost $1000, can only provide about 200 N/M of stopping torque. Therefore, even with the combination of the motor and an expensive electromagnetic brake, the system of the '095 patent would not appear to stop the blade in {fraction (1/200)}th of a second. Moreover, the system of the '095 patent relies on switching of relays to control actuation of the brake and motor. However, relays normally take 5-15 milliseconds to operate, so it is unlikely the electromagnetic brake or the motor braking could even be engaged within {fraction (1/200)}th of a second, let alone stop the blade.
As another example, U.S. Pat. No. 3,785,230 to Lokey describes a system for use on a hand-held circular saw for stopping the blade in the event a user's hand comes in dangerous proximity to the blade. This system uses a solenoid to shift a pair of cams into contact with opposed points on the side of the blade.
Due to the deficiencies with existing brake systems, it would be desirable to have a safety system that could be actuated quickly to protect a user against injury from a cutter.